1. Field of the Invention
The present invention relates to an oxygen sensor for detecting the concentration of oxygen contained in exhaust gas.
2. Description of Background Art
Most oxygen sensors are used for an air-fuel ratio control in industrial furnaces, boilers, and internal combustion engines.
Oxygen sensors, which make use of the principle of a cell, are classified into two types: one being intended to measure an electromotive force generated by chemical reaction of a cell portion of the oxygen sensor, and the other being intended to measure a change in conductance caused by applying a voltage to a solid electrolyte of a cell portion of the oxygen sensor. The former oxygen sensor is known, for example, from Japanese Utility Model Laid-open No. Hei 3-2256 entitled xe2x80x9cOxygen Sensor for Internal Combustion Engine.xe2x80x9d The configuration of the prior art oxygen sensor disclosed in this document will be described below. Referring to FIG. 1 of this Japanese Utility Model, a protector for protecting a sensor element unit 5 (the following reference numerals are used in the Japanese Utility Model application) is of a double cylinder structure having an outer cylinder 21 and an inner cylinder 22. Exhaust gas inlet holes 21a and exhaust gas introduction holes 22a are formed in peripheral walls of the outer cylinder 21 and the inner cylinder 22, respectively (see FIG. 2), and an exhaust gas outlet hole 22b is formed in a leading end plane of the inner cylinder 22.
Exhaust gas flows in the outer cylinder 21 through the exhaust gas inlet holes 21a. The flow of the exhaust gas is turned in a gap between the outer cylinder 21 and the inner cylinder 22 to be mixed with each other. The mixed exhaust gas flows in the inner cylinder 22 through the exhaust gas introduction holes 22a toward an electrode plane 5a having a directivity, and is then discharged from the exhaust gas outlet hole 22b. The exhaust gas is impinged on the electrode plane 5a along a specific direction irrespective of the orientation of the electrode plane 5a, so that the oxygen sensor can exhibit a constant oxygen concentration detecting performance.
The prior art oxygen sensor having the above structure, however, has problems. Since one side edge of each of the exhaust gas inlet holes 21a of the outer cylinder 21 is inwardly bent, the shape of the hole 21a is complicated, to increase the machining cost of the holes 21a. 
Since the gap is large enough to allow turning of exhaust gas therein between the outer cylinder 21 and the inner cylinder 22, the outside diameter of the outer cylinder 21 becomes large and thereby the size of the protector is increased. Also since the flow of exhaust gas is turned to be mixed with each other, it may take an excessive time until the exhaust gas reaches the electrode plane 5a. 
The prior art oxygen sensor has another problem in that the inner space of the inner cylinder 22 on the electrode plane 5a side (front side) is smaller than that on the opposed side (back side), so that if exhaust gas flows in the inner cylinder 22 from both the sides as shown in FIG. 3, the exhaust gas exhibits a high fluidity on the back side while it exhibits a poor fluidity on the front side, that is, on the electrode plane 5a side, whereby the responsivity of detection becomes poor. The prior art oxygen sensor, therefore, has room for improvement in terms of responsivity of detection. Since the responsivity of an oxygen sensor exerts a large effect on the performance of an exhaust gas purifying system, it is expected to improve the performance of the oxygen sensor more than ever.
An object of the present invention, therefore, is to provide an oxygen sensor excellent in responsivity of detection.
To achieve the above object, according to the present invention, there is provided an oxygen sensor mounted in an exhaust passage of an engine for detecting an oxygen component contained in exhaust gas, including: a sensor element unit including a plate-like first electrode; a second electrode disposed on one side of the first electrode with a solid electrolyte layer put therebetween; and a third electrode disposed on the other side of the first electrode with a solid electrolyte layer put therebetween, thereby detecting migration of oxygen ions between the first electrode and the second electrode and migration of oxygen ions between the first electrode and the third electrode.
Since the second electrode is disposed on one side of the plate-like first electrode with a solid electrolyte layer put therebetween and the third electrode is disposed on the other side of the first electrode with a solid electrolyte layer put therebetween, both the sides of the sensor element unit function as detecting planes. This means that the directivity of the oxygen sensor extends in two directions. Since either the second electrode or the third electrode faces to the flow of exhaust gas, the oxygen sensor of the present invention can be brought into contact with exhaust gas earlier than the prior art oxygen sensor does. Accordingly, the response time of the oxygen sensor of the present invention becomes shorter than that of the prior art oxygen sensor.
According to the present invention, in addition to the configuration of the invention described above, a plate heater having an opening portion allowing permeation of the oxygen ions therethrough is interposed in at least one of the spaces wherein one of which is between the first electrode and the second electrode and the other of which is between the first electrode and the third electrode.
Since the opening portion allowing permeation of oxygen ions therethrough is formed in the plate-like heater, if the plate-like heater is disposed on the second electrode side, oxygen ions migrate from the second electrode to the first electrode through the opening portion, to generate a current; while if the plate-like heater is disposed on the third electrode side, oxygen ions migrate from the third electrode to the first electrode through the opening portion, to generate a current. As a result, although the plate-like heater is provided in the sensor element unit, any reduction in detecting performance is not reduced.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.